Gas turbines with sequential combustion have been successful in commercial operation for some time now. In them, compressed air is combusted with fuel in a first combustor and a first turbine, referred to as the high-pressure turbine, is exposed to admission of hot gases. The temperature of the hot gases which discharge from the high-pressure turbine is increased again in a second combustor as a result of renewed addition of fuel and its combustion, and a second turbine, which is referred to as the low-pressure turbine, is exposed to admission of these hot gases.
Compared with conventional gas turbines with only one combustor, they are characterized by the additional degree of freedom of a separate fuel control for the first and second combustors. This furthermore offers the possibility of first of all putting into operation only the first combustor and engaging the second combustor only in the case of higher load. This enables a flexible operating concept with good emissions behavior over a wide operating range of the gas turbine.
In recent years, the main focuses of development were the reduction of NOx emissions and higher part load efficiency.
Gas turbines with sequential combustion, which are operated according to known methods, as are described in EP 0718470, for example, have very low NOx emissions and can achieve excellent part load efficiency.
The aforementioned known operating concepts, however, at low part load, especially within the range of about 20% to 50% of the relative load, can lead to high CO (carbon monoxide) emissions.
These high CO emissions are typically created at low part load by the second combustor of a gas turbine with sequential combustion. Conventionally, the second combustor is ignited at low part load if the rows of variable compressor inlet guide vanes are closed and the hot gas temperature or turbine inlet temperature of the high-pressure turbine has reached an upper limit value. For ignition, the second combustor is supplied with a minimum fuel flow which is typically prespecified by the control characteristic of the fuel control valve. On account of the high exhaust temperature of the first turbine, self-ignition of the fuel flow which is introduced into the second combustor occurs. The fuel flow is increased via the load for load control. Providing the fuel flow is low, the temperature of the hot gases in the second combustor is not significantly increased. The reaction speed remains correspondingly relatively low and unburned hydrocarbons and CO may occur on account of the short residence time in the combustor. These occur especially in the case of lean combustion that is to say in the case of combustion with a high air ratio λ. The air ratio λ is the ratio of air mass actually available for combustion to the at least required stoichiometric air mass. It is also referred to as air coefficient, air ratio number, or excess air.
Within the limits of a flexible power plant operation, however, the possibility of running for longer operating periods at low part load is increasingly also required. A longer operation at low part load can only be realized if the CO emissions also remain at a low level.
One successful method to keep the CO emissions at partial load low is known from CH 700796 A and the parallel U.S. Pat. No. 8,434,312 B2 of the applicant. This method controls the air fuel ratio of the operative burners of the second combustor to keep the CO emissions low.